may be the thumbturn on the latch was removed, this would leave a hole which unless you had a chubb/star key you wouldnt be able to escape, thinking about it surely they had a window I am sure I could muster the strength to pull the cistern off the wall to smash it but then again suppose you just stay put! Luckily a few weeks ago when we were broken into they left us asleep!!
Our bathroom has a lock but the key has been lost years ago. Our house is an old one built before inside plumbing and the bathroom is a former bedroom. All the doors have locks on them, including the downstairs rooms. The outside loo has a suffolk latch. I think it all depends when your house was built.